DEARBORN, Mich., Oct. 9 -- On a day when stock indexes hit record highs, Republican presidential candidates gathered here Tuesday for a group defense of low taxes and free markets and warned that Democrats, particularly Sen. Hillary Rodham Clinton, pose the greatest danger to the nation's future prosperity.

Former senator Fred D. Thompson (Tenn.) finally joined his rivals in a televised debate, adding his voice to the chorus singing the praises of free trade, a reduction in regulation, private health care and reduced government spending. Like the others on the stage, he made his points by taking aim at the opposition party.

"When the Democrats start targeting the rich guy, if you're a middle-class guy you ought to run to the other side of the house, because you're going to get hit," Thompson said. The former "Law & Order" star appeared nervous as the debate began but seemed to grow more comfortable by the end of the two-hour event.

"I've enjoyed watching these fellas," he said when asked if he enjoyed himself. "I gotta admit it was getting a little boring without me."

The week-long spat between former Massachusetts governor Mitt Romney and former New York mayor Rudolph W. Giuliani spilled from the campaign trail onto the stage, with each accusing the other of failing to keep taxes low and to control spending.

Romney accused Giuliani of supporting a commuter tax in New York, prompting an annoyed Giuliani to retort that "the point is that you've got to control taxes. But I did it; he didn't. . . . Under him, taxes went up 11 percent per capita. I led; he lagged."

Romney refused to let the jab go by without a response, saying: "It's baloney. Mayor, you've got to check your facts. . . . I did not increase taxes in Massachusetts."

Clinton (N.Y.), the front-runner for the Democratic nomination, became a frequent target throughout the debate, as the leading Republicans competed to demonstrate their readiness to challenge her in a general-election contest.

Giuliani accused Clinton of being weak because she did not answer a question at a Democratic debate about the use of military action to prevent Iran from achieving nuclear capability. "Well, you've got to answer the question. The answer is yes, we would. Iran is a greater danger than Iraq."

Giuliani also accused the Democratic front-runner of economic pessimism, saying she favors "endless ways to spend" taxpayers' money and threatens the U.S. health-care system with her reform proposal. In a quip toward the end of the debate, he said, "I think there's a looming problem with Canada that you missed. If we do Hillary-care or socialized medicine, Canadians will have no place to go to get their health care."

Romney was even more aggressive in challenging Clinton's health-care plan, though in broad outlines it resembles the measure approved in Massachusetts while he was governor.

"The way we improve something is not by putting more government into it -- of course, that's what Hillary Clinton wants to do," he said. "Hillary-care is government gets in and tells people what to do from the federal government's standpoint."

Answering a question about taking military action against Iran without consulting Congress, most of the candidates took a hard line, with Sen. John McCain (Ariz.) saying that while getting the approval of Congress is always best, "if the situation is that it requires immediate action to ensure the security of the United States of America, that's what you take your oath to do."

Romney offered a legalistic answer likely to invite criticism, saying that "you sit down with your attorneys" first, "but obviously the president of the United States has to do what's in the best interest of the United States to protect us against a potential threat."

Its purpose, according to the sponsors, was to focus the candidates' attention on economic issues. Dearborn, home to Big Three automaker Ford, was "the perfect backdrop to address the concerns of a rapidly changing workforce," said Michigan Republican Party Chairman Saulius "Saul" Anuzis.

But the candidates frequently addressed the questions more globally, vigorously defending free-trade agreements that they said provide American companies expanded opportunities to sell their goods and services abroad.

"We can't say, 'Because these agreements weren't perfect, because they have problems, because they have issues, we're going to turn our back on free trade,' " Giuliani said.

Many of the candidates even found kind words for organized labor, praising the work of unions. Brownback fondly recalled his mother's membership in the postal workers union.

"She called herself a postal packin' grandma for a good period of time," he said. "And it helped her on health care."

Asked to name a good union, Romney quickly offered the carpenters union and then prompted chuckles when he added: "I'm probably not going to name specific bad unions."

Huckabee urged his party to pay more attention to issues of economic insecurity and to confront the reality that many Americans do not believe international trade agreements have been good for the country.

"This party is going to have to start addressing it or we're going to get our britches beat next year," he said.

Thompson's participation in the debate did not substantially change the dynamic from that of past events. But questioners targeted him with the most questions and even quizzed him on the name of the Canadian prime minister. (He answered correctly, quickly giving Stephen Harper's last name.) He was the subject of some gibes, however. Romney said the series of six debates reminded him of Thompson's "Law & Order" television series: "A huge cast, the series seems to go on forever, and Fred Thompson shows up at the end."

Thompson joked: "And to think I thought I was going to be the best actor on the stage."

Paul and Tancredo were the lone dissenters on most issues. Paul decried the war in Iraq and proclaimed talk of attacking Iran as "war propaganda" that "is a road to disaster for us as a nation." Tancredo turned almost every question he was asked into an argument against illegal immigration.

And neither would promise to support the GOP's eventual presidential nominee.

"Not right now, I don't," Paul said. "Not unless they're willing to end the war and bring our troops home."